Digital integrated circuit chips are composed of many millions of gates that make up various functional components on a chip such as flip-flops, multiplexers, logic circuits, etc. A given chip design may have thousands of flip-flops scattered throughout the chip.
In order to effectively and efficiently test a given chip, certain test features are typically incorporated into the chip design for testing purposes. Before a chip is actually taped out and manufactured, the chip design is first simulated in software using various simulation tools such as, for example, a Verilog Test Bench. By simulating the design of the chip, the design features of the chip may be thoroughly tested before the expense and time of actually manufacturing the chip is incurred.
Pattern verification is a critical phase in testing of chips. A scan pattern is a digital string of binary ones and zeros that may be shifted through a scan chain of flip-flops in the chip design. Every scan pattern cycle is composed of two phases. The first phase is the load_unload phase where new data is shifted into the scan chains of flip-flops. The second phase is the capture phase where the data is captured into the flip-flops by applying a clock pulse.
Typically, the flip-flops in a digital integrated circuit design are designed such that they have normal data inputs and outputs (D and Q) and test inputs such as TI (test data input) and TE (test enable input). During simulation, the flip-flops may be placed in the test mode by enabling the TE input. Data may then be clocked into the flip-flops through the TI input instead of the normal D data input. During testing, the flip-flops of the chip are chained together to form multiple scan chains. The output Q of a given flip-flop is connected to the input TI of a next flip-flop. Each scan chain may comprise, typically, 5000 to 10,000 flip-flops.
The length of the load_unload phase is equal to the length of the longest scan chain of flip-flops. In multimillion gate designs, the longest chain may have thousands of flip-flops. Most of the time, simulating the scan patterns through the scan chains is spent shifting the data into and out of the scan chains. No matter what the length of any given scan chain, the load_unload phase is still equal to the length of the longest scan chain of flip-flops. If one scan chain is very short and another scan chain is very long, the longest scan chain drives the time it takes to perform the scan test since the scan chains are typically clocked in parallel.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with the present invention as set forth in the remainder of the present application with reference to the drawings.